Carrera Stalling When Stopping - Idle Switch? Air Control Valve?

[RandalJ]

I have an '86 Carrera that runs perfectly, except often when I come to a stop in hot weather (I'm in Phoenix), the RPMs drop to zero. Other times they drop almost to zero and "bounce" back to normal idle speed. Other times the RPMs do not drop much, if at all, below normal idle speed.

I have looked in the archives, and it looks like the idle switch and air control valves top the list as suspects. I have not totally ruled out a vacuum leak somewhere or air/fuel mix, but since the driveability is so good otherwise (acceleration strong, idle normal and very stable otherwise), I suspect vacuum and mix are OK.

The situation seems to be aggravated by hotter weather. It began to get bad when temps started hitting the 90s and above here in Phoenix. A/C on makes it understandably worse, and I remember having the problem last September when I first bought the car (September is still hot here). The stalling went away last fall, and came back this late spring.

The worst possible conditions to bring this on seem to be when I pull out of work in the PM near the hottest point of the day with A/C on. Engine is "cold" (though probably at 100-110 deg F ambient), ruling out O2 sensor I think. It starts right up, and holds a steady idle, but when I drive a few hundred feet and stop at a light, as soon as I push in the clutch, the tach dives for zero. Even with the A/C off, it will more often than not die.

It will start right back up, and hold a good idle, no problem - until the next stop.

As one other person has pointed out, you can prevent this somewhat by waiting til the very last moment before depressing the clutch.

Once fully warmed up, it seems to do this less often, but sometimes I end up coasting into my garage when I get home!

So, it seems to always do it the first few stops when I leave work, and it becomes intermittent (maybe 20-30% of the time) after that.

In the morning, when it is cooler, it still does this but not as frequently. Seems to be ambient temp sensitive.

It would be nice to have a spare throttle position switch ($50) to just throw in and test. And an air control valve ($130) if that doesn't work. I did take the valve out and tested it per the Bentley book, but couldn't determine whether the spindle inside was moving as freely as it should. Resistances measured high (80 ohms across instead of 40), but in that range I figured there is probably alot of slop in the specs (maybe not?). Checked the idle stop switch, and it seems to be clicking. Will check the switch's gap next.

I'm just wondering if anyone else has seen this on a 3.2/Motoronic system, and come up with the definitive fix.

I'm trying to figure out what affect high ambient temps could have on either of these components - or on anything, that engine heat alone would not cause in the cooler months...

The hot outside air passes through both the air valve and throttle sensor. I'm thinking that if either of these are marginal, perhaps they could be binding intermittently.

I'd also like anyone's opinion whether I am right to rule out vaccum leaks or bad air/fuel mix. I guess my main argument against these is that it idles just fine in all weather, it pulls strongly in all weather, with no missing or rough spots. Since the ACV and idle stop switch are specifically there to control idle in throttle-off conditions, I just have to think it is one of them.

Thanks in advance for any opinions.

[Jeff]

Randal,

First off, welcome to the board.

Have you checked the cylinder head temp sensor?

Where in Phoenix are you?

[RandalJ]

Ahwatukee area. If it was the cyl head temp sensor, wouldn't I have other driveability problems?
Thanks

[Jeff]

Not necessarily. It sounds like you DME is thinking your car is much hotter than it is since it happens during hotter weather.

You may also want to check your speed and reference sensors, if they are within specs check them visually. They are routed over the cat and are subjected to a lot of heat.

Hey we are neighbors.

[DJB]

I have excatly the same with my 86 - except when its cold. May be it is the temp sensor?

[RandalJ]

Checked the cylinder head temp sensor. About 200 ohms, which is about right for a still hot engine (shut off a half hour ago). I'll have to check again when cool. The idle off switch checked out OK. Haven't checked the speed and ref sensors yet.
Thanks

[mm86911]

Had a similar problem this year - got progressively worse over time. My idle issue was would resolve itself once engine was fully warm - but over it would continue even then. Idle would roll a bit even after fully warm. Removed the air idle bypass valve, cleaned it thoroughly, and problem solved.

[RandalJ]

mm - how did you clean it? I've had it out, but it it did not look as though it could be disassembled. Have you had lasting results, or is it too short of a times since you did it to tell?

One thing I noticed on mine, the valve was more of a spindle that would turn, but not move much lengthwise. I was expecting a piston, from the description in the Bentley book.

Is the moving part supposed to turn or slide to modulate the air?

It does hum like it's supposed to.

I noticed a slight coating of engine oil on one end (can't remenber which end), from one of the hoses. Is this normal? Oil level is never above half when hot.

Thanks

[greglepore]

You can pry up the little tabs on the icv and take it apart.

I'd try opening up your idle air bypass screw a bit and see if that doesn't solve the problem. It did for me.

[mm86911]

While I can't remember teh internal workings the cleaning was rather simple - brake cleaner followed by a quick shot of WD-40 for a little lubrication. Was rather dirty (oil residue) so you may also want to clean the tubing connecting to the throttle body. Operating perfectly for 6 months now.

[pirin911]

I have the same problem, I changed both the cilinder head temperature switch and the air control valve and nothing happened. I still have the problem. I opened the idle screw a bit in the throttle body and it alleviated a bit but sometimes after a hard run the RPM's stay above 1000, so I backed it up again.

[RandalJ]

I took apart the air control valve. Looks like the brushes have worn a big pit into the shaft. There are three brushes, one for each connection on the harness. It is the front-most (toward front of car, or furthest from you when you are looking in from the back) connection that connects through the brushes to this pitting. The surfaces associated with the other brushed are relatively wear-free. I will replace the whole valve today and let you know what happens.

[greglepore]

Randall-mine looked the same way, but I just put it back together and used it. Let us know if there's any difference when you replace it.

[RandalJ]

Since I could not get a replacement today, I did put it back together, blew som brake cleaner through the tube, and squirted a tiny bit of WD40 on the spindle. I started it up, and the problem was still there. I have come to realize that I don't even have to drive it. If I start up, it idles just fine. If I rev the engine to 2000 or so, and let off, it dies. It will do this with the A/C on or with it off, but its a bit worse with it on. As I mentioned before, sometimes it goes almost to zero, then recovers. It's almost as though the valve is working, but is not not turning on fast enough. Every time I start the car (A/C on or off) it goes right to idle speed like its supposed to, even if it died right before.

[RandalJ]

Checked so far or eliminated as a cause:
1. Cylinder head temp sensor - OK - 1.4k cold at about 85deg ambient, and around 200 ohms when warm.
2. O2 sensor - ignored by ECU when engine is cold, so is not a factor.
3. Idle-off switch is clicking properly.
4. Had full tune-up last fall, about 6k miles ago.
5. Resistance measurements on idle air control valve are in spec, but there is a big pit on one of the contacts.
6. I put everything back, and started the car without the IACV connected at the harness. Ran a little rough. I plugged it in, and heard a strong hiss as the valve opened and smoothed out the idle.

Any ther ideas on what to check during throttle off conditions?

Thanks

[Jeff]

Randal,
I just took my air control valve apart and cleaned the contacts, etc... Looking at mine, it seems like you could make a more gradual entrance and exit for the contact that has worn the groove. And also bend the contact so that it puts more tension on the spindle ( so it stays in contact even when it falls into the groove). I'm guessing the contact is hanging up on the groove from time to time and giving you the low/no idle.

Be careful to mark corrolation between the two pieces of the valve and test the valve before you bend the tabs over. You will get different resistance if it turns.

If you need P.M. me and we can get together and I will show you what I mean.

[Jeff]

Randal, You have a PM

[pirin911]

I asked Todd from Protomotive about this problem and he told me the following:

You just need to either open the throttle plate a bit, or the idle air bleed screw.
The idle should be at 850-880 rpm's. Try to set the idle speed hot, but either opening the air bleed or the throttle plate till you just exceed the idle rpm. Then, turn it back down just till it hits it. Right now, you're just relying the the idle air compensator motor too much, and with the lightweight setup in the clutch, it can't recover quickly enough. By opening up the bleed screw or throttle plate you'll not allow the manifold to go into such a vacuum. This way, it's never too far from recovering.
You can also test the idle just sitting there. From the engine bay, just blip the throttle. It should come back to idle with no droop and catch strong. if it droops, open the screw or the throttle plate. if it comes down and catches strong, leave it alone. If it bounces, you're too high.
The factory way is to jump terminals B and C on the test connector and set the idle speed in this test mode. That works well also.

[cary]

I'd change the DME relay. Making sure the terminals are tight and clean.
Throw the old one in the glove box for a spare.

[RandalJ]

I just wanted to let everyone know that today I have discovered a much stronger correlation than I previously thought between high ambient temps and this off-throttle stalling problem

Last week, humidities were relatively high, so morning low temps were in the 90's (relatively high). I saw the stalling frequently during the morning warm-ups last week, but just assumed that the problem was getting worse in general.

This week, starting about Monday, it was much drier, hence cooler (upper 70's or low 80's) in the mornings. No stalling at all. Not even a hint of the RPMs wanting to dive to zero.

Went out at lunch, whe it was a dry 100 deg outside, and it stalled several times within minutes of starting up. Got better as engine warmed up.

After work, 108 outside, same thing.

This AM, cool again. No stalling. Same as yesterday morning.

So it appears that above about 90-95 deg ambient, during warm-up, there is something not working right. It appears that the idle air control valve is probably working (though I'm gonna replace it anyway). I base this on the off-throttle idle being OK at lower ambient temps and less of a problem (but not totally gone) at higher engine temps. That is, the temp is not affecting it directly, otherwise engine heat would affect it same as ambient heat.

It seems that either a sensor is not detecting ambient air temp above this range, or something is not reacting to it during warm-up properly.

I think I will also try the new relay (cheap) and look really closely at the air idle mix adjustment (free).

I did verify that the cyl head temp sensor was working, but only at the plug on the engine. I maybe should check to see if the signal is getting all the way back to the ECU.

Thanks again for all your suggestions.